Listen to this clip of 'Be Bop A Lula'. What effects can you hear?
This effect is called 'slapback'. It is a short, single delay applied to the original signal at about 40-120 milliseconds. Nowadays, it is simply and easily achieved in freely available software plugins. In the 1950s however, delay was achieved using tape machines. You will often see presets on digital delay plugins called 'tape delay'.
This video is an excellent example of how reel to reel tape machines were used to create slapback (and other delay effects).
1950s amplifiers used valves to amplify the weak alternating signal from a guitar lead. The following video is truly ancient (although with modern annotations)but it is one of the clearest descriptions of how valve amps actually work that you could ask for!